This invention relates to an apparatus and method for measuring spectral characteristics of a measurement sample containing a fluorescent substance.
In general, a visual characteristic of a sample containing a fluorescent substance (hereinafter, "fluorescent sample") can be expressed by a ratio of light emitted from the illuminated sample and light from a completely diffusive reflecting surface illuminated under the same conditions, i.e., a total spectral radiance factor. The total spectral radiance factor Bt(.lambda.) is given by the following Equation (1): EQU Bt(.lambda.)=Br(.lambda.)+Bf(.lambda.) (1)
wherein Br(.lambda.), Bf(.lambda.) denote a reflected spectral radiance factor by reflected light components from the fluorescent sample and a fluorescent spectral radiance factor by fluorescent components from the fluorescent sample.
Particularly, a fluorescent sample containing an important fluorescent brightener is usually excited according to an excitation efficiency P(.mu., .lambda.) by an exited light of wavelength .mu. in an ultraviolet spectrum. Accordingly, the fluorescent spectral radiance factor Bf(.lambda.) at wavelength .mu. depends on a spectral intensity I(.mu.) of an illumination light as defined by the following Equation (2): EQU Bf(.lambda.)=.intg..sub.UV I(.mu.)P(.mu., .lambda.)d.mu./Sn(.lambda.)(2)
wherein Sn(.lambda.) denotes a relative spectral intensity of a standardized illumination light.
If S.sub.W (.lambda.), R.sub.W (.lambda.) denote spectral intensities of an emitted light and a reference light when a nonfluorescent white standard sample having a known spectral reflectance W(.lambda.), and S(.lambda.), R(.lambda.) denote spectral intensities of an emitted light and a reference light when the sample is measured by a colorimeter, the total spectral radiance factor Bt(.lambda.) is given by the following Equation (3): EQU Bt(.lambda.)=W(.lambda.).multidot.(S(.lambda.)/R(.lambda.))/(S.sub.W (.lambda.)/R.sub.W (.lambda.)) (3)
However, for the aforementioned reason, the spectral intensity of the illumination light needs to correspond with the spectral intensity of the light used for a supposed colorimetry in the case that a fluorescent sample is measured.
The lights used for the colorimetry include D50, D55, D75 (daylight), F1, F3, F11 (fluorescent lamp) as well as standard lights such as D65 (daylight) and A (incandescent light source) whose spectral intensities are defined by the CIE (Committee on the International Illumination). For fluorescent samples, standard light D65 is usually used.
However, since it is very difficult to obtain an illumination light source approximate to the standard light D65, a relative intensity in the ultraviolet spectrum has conventionally been adjusted according to a method as shown in FIG. 7 ("Assessment of Whiteness and Tint of Fluorescent Substrates with Good Instrument Correlation" by Rolf Griesser, "The Calibration of Instruments for the Measurement of Paper Whiteness" by Anthony Bristow in "COLOR Research and Application, Vol. 19, No. 6, Dec. 1994).
In FIG. 7, a fluorescent sample 1 is placed on a sample aperture 21 of an integrating sphere 2. A light source 101 such as a xenon lamp having a sufficient intensity in the ultraviolet spectrum is driven by a light emission circuit 104 and a beam 102 is introduced into the integrating sphere 2 through an aperture 23. Here, an ultraviolet cutoff filter 103 is inserted so as to partially cut off the beam 102. Accordingly, the beam having transmitted through the ultraviolet cutoff filter 103 has ultraviolet components removed therefrom. The degree of insertion of the ultraviolet cutoff filter 103 is adjustable so as to adjust a relative ultraviolet intensity of the illumination light.
The beam 102 introduced into the integrating sphere 2 undergoes a multiple diffusion/reflection therein to become a diffused light and illuminates the fluorescent sample 1. Light comprised of components in a specified direction emitted from the illuminated sample 1 is incident on a spectral device 105 for the sample through an observation aperture 24, whereby a spectral intensity S(.mu.) is detected. Simultaneously, a beam 62 having substantially same spectral intensity as the illumination light of the fluorescent sample 1 is incident on a reference fiber 61 to be introduced to a spectral device 106 for the reference, whereby a spectral intensity R(.lambda.) is detected.
In order to adjust a relative intensity in the ultraviolet spectrum, a nonfluorescent white standard sample 12 having a known spectral reflectance W(.lambda.) is first measured, and then a standard fluorescent sample 13 having one known color value of those obtained when being illuminated by a standard light for the colorimetry, e.g., ICE whiteness is measured. The degree of insertion of the ultraviolet filter 103 is so adjusted that the CIE whiteness calculated from the total spectral radiance factor Bt(.lambda.) obtained from Equation (3) agrees with the known CIE whiteness. At this time, Equation (2) can be rewritten into the following Equation (4): EQU Bf(.lambda.)=.intg..sub.UV .alpha..multidot.I(.mu.)P(.mu., .lambda.)d.mu./Sn(.lambda.) (4)
wherein .alpha. denotes an attenuation coefficient in the ultraviolet spectrum.
A known apparatus is provided with a first light source for irradiating a beam containing light components in the ultraviolet spectrum and a second light source for irradiating a beam containing light components outside the ultraviolet spectrum (see U.S. Pat. No. 5,636,015). In this apparatus, the spectral intensity of the light emitted from a sample upon being illuminated by the illumination lights from the first and second light sources are weighted and the spectral intensities of the illumination lights from the first and second light sources are weighted as seen in Equation (8) of this publication.
In the prior art shown in FIG. 7, the relative ultraviolet intensity of the illumination light can be so adjusted as to give the same CIE whiteness as when illuminated by the standard illumination light for the colorimetry for the same fluorescent sample as or the one similar to the standard fluorescent sample used for the adjustment of the relative intensity in the ultraviolet spectrum.
However, a highly accurate measurement may not be conducted since other color values such as tints at that time may not necessarily be same. Further, since the measurement and the movement of the filter need to be repeated for the intensity adjustment in the ultraviolet spectrum, it takes a time to obtain a measurement result.
As described above, in the apparatus disclosed in U.S. Pat. No. 5,636,015, the spectral intensity of the light emitted from the sample and that of the illumination light illuminating the sample are weighted, respectively. Accordingly, if the spectral intensity of the illumination light varies, the weighted result varies even if the same sample is used. In other words, relative variations of the spectral intensities of the illumination lights from the first and second light sources adversely affect the total spectral radiance factor.